1. Field of the Invention
The present invention generally relates to steady rests employed to rotatably support a cylindrical workpiece while undergoing a precision machining or grinding operation. More specifically, this invention relates to a multi-point true centering steady rest having an internal centerline adjustment mechanism that enables the steady rest to more precisely support a rotating cylindrical workpiece at its dynamic working center during a machining or precision grinding operation.
2. Description of the Prior Art
Steady rests are commonly used in manufacturing to support rotatable cylindrical workpieces during machining or grinding operations. A steady rest provides an accurate positioning of a cylindrical workpiece relative to a grinding wheel or a cutting tool while allowing the workpiece to rotate. In addition, a steady rest serves two important functions. The first function is to prevent deflection of a rotating cylindrical workpiece at the point of grinding or a cutting operation thereon, a condition which becomes more likely as the length of the cylindrical workpiece increases. The second function is to provide a stabilizing effect on a rotating cylindrical workpiece to improve the precision and uniformity of a machining or grinding operation on the workpiece. Accordingly, the use of a steady rest or similar device is essential for accurately and precisely machining or grinding rotating cylindrical workpieces.
Steady rests which exemplify the state of the art are disclosed in U.S. Pat. Nos. 4,399,639, 4,647,097, 4,650,237 and 5,508,468 to Lessway, all of which are assigned to the assignee of the present invention. The above steady rests and others known in the prior art generally have at least two points of contact for supporting a workpiece around its perimeter. A steady rest having three support arms, such as that disclosed in U.S. Pat. No. 5,058,468 to Lessway, is particularly desirable for its ability to accurately support a workpiece through its longitudinal axis, or centerline.
The steady rest disclosed in U.S. Pat. No. 5,058,468 achieves a three-point centering and supporting action in the following manner. The supporting action is provided by a center workpiece contact member and a pair of support arms, each having a side workpiece contact member. The support arms are operatively attached to an operator body slidably mounted within a housing. The center workpiece contact member is disposed on one end of the operator body, adjacent and between the support arms. The supporting action of the support arms may be produced by a combination of rollers and fixed guide pins, or by flat guides, which selectively follow a pair of camming contours in the housing walls, as described in said U.S. Pat. No. 5,058,468. As the operator body is moved within the housing toward a workpiece, the support arms first move parallel with the longitudinal axis of the operator body. Once alongside the workpiece, the support arms, in cooperation with the camming contours, move laterally to converge upon the workpiece in a manner which operatively provides a centering and supporting engagement with the workpiece.
The demands of current and future manufacturing and machining technologies have resulted in many applications requiring higher degrees of precision than ever before, with tolerances on such items as camshafts, crankshafts and turbine shafts approaching as little as 1 micron (0.000039 inches). Whereas prior art steady rests have performed very well under typical circumstances, dynamic and transient conditions associated with turning and grinding operations have prevented these steady rests from being used for higher manufacturing tolerances without time consuming adjustments.
As an example, a typical manufacturing procedure is to use a master setup bar to establish what is termed the static centerline for a cylindrical workpiece. The static centerline serves to establish the axis of the workpiece to which the machine and workpiece are originally set for the desired operation. The steady rest is then secured to the machine in a position that will support the workpiece through its static centerline. Thereafter, the workpiece is ground or turned until, according to the original settings, the desired diameter is obtained. The workpiece is then removed and the diameter checked with an off-line gauge. Under standard tolerance conditions, the ability of the steady rest to stabilize and accurately support the workpiece relative to the grinding wheel, or tool, would be sufficient to achieve the desired diameter or the workpiece. However, where tolerances demand greater precision, the desired diameter may not be obtained, requiring the workpiece to be either scrapped (if undersized) or reworked (if oversized).
The cause for the oversize or undersize condition is two-fold. First, the workpiece experiences a dynamic shift in its centerline as a result of rotational forces and transient thermal conditions as the room, machine, grinding wheel or tool, and workpiece reach their respective steady-state temperatures. The resultant centerline is referred to as the dynamic centerline of the workpiece. Secondly, the workpiece's centerline undergoes a dynamic shift caused by the force applied by the grinding wheel, or tool, to the workpiece during the actual grinding or machining operation. This resultant centerline is referred to as the dynamic working centerline. The degree of departure from the dynamic centerline is not readily predictable, in that it is influenced by such diverse factors as the lubricity of coolants used, the grinding wheel dress condition, different grit sizes in the grinding wheel, and the material of the grinding wheel, or the sharpness of the cutting tool.
As a result, the dynamic working centerline may be sufficiently displaced from the initially set static centerline, such that the diameter of the workpiece is not within the tolerance band required. Regardless of whether an oversize or undersize condition results, an adjustment in the position of the steady rest, relative to the workpiece, must be made in an effort to seek the dynamic working centerline of the workpiece. Heretofore, it has been necessary to physically move the steady rest, by loosening its mounting bolts, then repositioning the steady rest using dial indicators to gauge the repositioning of the steady rest relative to the grinding wheel or cutting tool. Such a task is time consuming and not always sufficiently accurate to correct for the oversize or undersize condition. Accordingly, the above procedure must typically be repeated until the dynamic working centerline has been found.
As can be appreciated from the above, it would be desirable to incorporate within the steady rest a mechanism for adjustment, such that the steady rest could be displaced relative to the workpiece, without the need for the steady rest to be loosened and physically repositioned. The steady rest designs noted above are limited in that the action of the support arms is defined by camming contours which are formed in the interior walls of the steady rest housing. Because the interior walls of the prior art steady rest housings are stationary, such steady rests are limited to an external adjustment means for repositioning their housings. Such an external adjustment means would be undesirable from the standpoint of precision in making fine adjustments.
Accordingly, what is needed is a steady rest having an internal centerline adjustment mechanism for adjusting the action of the steady rest support arms, and which adjustment mechanism is capable of making fine adjustments relative to a workpiece. It would be additionally desirable if the internal adjustment mechanism could be capable of selectively operating on each support arm independently, to accommodate any eccentric shift of a workpiece relative to the steady rest and the grinding wheel, or cutting tool.